[Carannillion]

Ah yes, good one, Iathspell.

"In my experience, there is no such thing as luck."
- Obi-Wan Kenobi

Seriously, I said "...not really just pure luck...". I think that yes, we have to give Fortuna her well-earned attention in some cases, but we can not blame it all on her. Like you said, "Or are there other powers at work of which we don't know (yet)?"
Take a look at your own signature... Sometimes I get the feeling that there is even more to Gandalf than we already know and assume. At least he knows a lot more than we can possibly imagine - of course, he was Olórin in the Undying Lands, most learned of all the Maia. Any other powers? I am not sure if I want to speculate in that, but I will not say that it is impossible that the Valar - or even Ilúvatar - sent more than just the Istari to help the peoples of Middle-Earth.

Now, the orc-running-episode.
The orcs were, as mentioned, Sauron's servants. They were like him. They had no reason to believe that anyone in their ranks would be anything else than orcs. They were from different places, both big and small orcs, and so Sam and Frodo in good disguises should not really pose any problem. These orcs were also tired, and so even if they did suspect any monkey business, they would probably just let it pass, to aviod any extra trouble or stress. They did not know - like the border guards did - that a pair of brave hobbits were headed to Mount Doom with the one thing which could destroy everything. They probably did not even know anything about the Ring at all.

Luck? Perhaps. A little bit, yes.
Excellent planning from Gandalf? Oh, yes indeed.

Gandalf's sewing toghether of things, to make the war come to a climax just when Frodo and Sam would need it the most was brilliant. The parting of the fellowship - or any such event which would lead to Frodo having to do his quest with fewer or no followers - had probably been calculated.
The Nazgûl outside Mordor in the moment it all went down, their leader dead, and so on...